<html>
<head><title>Calculator</title></head>
<body>

<h1>Calculator</h1>
(Ver 1.3 3/31/2011 by Chu-Cheng Hsieh)<br />
Type an expression in the following box (e.g., 10.5+20*3/25).
<p>
<form method="GET">
<input type="text" name="expr"><input type="submit" value="Calculate">
</form>
</p>
<ul>
<li>Only numbers and +,-,* and / operators are allowed in the expression.
<li>The evaluation follows the standard operator precedence.
<li>The calculator does not support parentheses.
<li>The calculator handles invalid input "gracefully". It does not output PHP error messages.
</ul>
Here are some(but not limit to) reasonable test cases:
<ol>
  <li> A basic arithmetic operation:  3+4*5=23 </li>
  <li> An expression with floating point or negative sign : -3.2+2*4-1/3 = 4.46666666667, 3+-2.1*2 = -1.2 </li>
  <li> Some typos inside operation (e.g. alphabetic letter): Invalid input expression 2d4+1 </li>
</ol>

<?php  $expr=$_GET["expr"];  
if(preg_match("/(([\/][0])|([\/][0]\.0+))$/",$expr))
  { echo $expr;}
else if (preg_match ("/^([-+]?[0-9]+\.?[0-9]*)(((([\*\/][+-]?)|([+][-]?)|([-][+]?))[0-9]+\.?[0-9]*)*)$/", $expr))  {
    eval("\$ans=$expr;"); echo $expr."=".$ans; 
} else {
    echo "Invalid input expression ".$expr . ".";
}





?>



</body>
</html>

